1. Field of the Invention
This invention relates generally to methods of terminating undesirable gas migration in wells, and more particularly, to such methods which are relatively simple and inexpensive to carry out.
2. Description of the Prior Art
After a well bore is drilled by rotary drilling wherein a drilling fluid is circulated through the well bore, the circulation of the drilling fluid is stopped and a production casing string is typically run into the well bore. After the casing has been run, primary cementing is performed. That is, the string of casing disposed in the well bore is cemented therein by placing a cement slurry in the annulus between the casing and the walls of the well bore. The cement slurry is permitted to set into a sheath of hard substantially impermeable cement in the annulus which holds the casing in the well bore and is intended to bond the casing to the walls of the well bore whereby the annulus is sealed.
Undesirable gas migration in a well is the migration of gas in the annulus from one or more pressurized gas formations or zones penetrated by the well bore during and after primary cementing. The gas migration can be between formations or zones, e.g., from a high pressure zone to a low pressure zone, or the gas migration can be from one or more gas zones to the surface.
It is widely believed that gas migration is caused by the behavior of the cement slurry during the transition phase in which the cement slurry changes from a fluid to a highly viscous mass having some solid characteristics. The transition phase starts when the cement slurry develops enough static gel strength to restrict the transmission of hydrostatic pressure over its column height, and ends when the cement slurry develops a gel strength which is sufficient by itself to prevent the migration of gas through the cement slurry. If the hydrostatic pressure exerted on one or more pressurized gas formations or zones by the cement slurry falls below the pressure of the gas in the zones, the gas enters the annulus and migrates through the cement slurry. The initial gas migration causes passages, e.g., flow channels or very small annular spaces between the casing, the cement column and the walls of the well bore known as "microannuli" to be formed. Such flow channels and microannuli remain after the cement slurry sets and undesirable gas migration continues.
While numerous techniques have been developed and used heretofore for preventing the formation of passages in the cement sheath in a well through which gas migration can occur, such techniques are not always successful and gas migration still results. Such gas migration often travels to the surface in wells which penetrate shallow gas zones.
The elimination of surface gas migration has heretofore been difficult, particularly in wells which penetrate shallow gas zones up-hole from the completed producing formation or formations. The shallow gas zones usually have very low permeability, and often include clays which swell upon contact with water thereby making it difficult to introduce cement for plugging the zones thereinto. Generally, the heretofore utilized techniques for eliminating undesirable surface gas migration have been unreliable, difficult to carry out and very expensive. Thus, there is a need for an improved relatively simple and inexpensive method of terminating undesirable gas migration in wells, particularly in wells where the gas migration originates from relatively shallow gas zones.